The present invention relates to an improved magnesium phosphate cement system, and in particular, to an improved magnesium phosphate cement system in which a set retarder is employed to produce a workable yet durable high strength patching system for use in roadways and other structure surfaces.
While the use of magnesium phosphate cement systems in general has been well known in the prior art, the use of such systems as a road patch has been severly limited because of the rapid setting of such systems. Usually magnesium phosphate cement systems have initial set times of less than 7 minutes (as measured by ASTM C-191) and may be as little as 4 minutes or less.
Certain retarders have been suggested for such patch systems, for example, oxy-boron compounds disclosed in U.S. Pat. No. 3,960,580. However, many of these prior art retarders seriously affect the strength and other characteristics of the resulting patch, making them undesirable for such use. In case of the oxy-boron class retarders, the amount of retarder which is generally employed--on the order of about one-half percent by weight--is so small that in practice there are problems in properly dispersing it throughout the mix. Amounts of oxy-boron retarder in excess of this greatly increase the initial set time beyond the desired 15 to 20 minute maximum (at room temperature). Should a road patch takes too long to set, it will not gain strength sufficiently fast, thereby resulting in early deterioration of the patch. Minimum compressive strength of the patch should be about 3000 psi after one hour. Other problems with prior art cement systems include problems associated with storage and use for example, poor flow characteristics and the necessity to mix more than two components together prior to use, which presents substantial inconveniences at a job site or other field application.
Bearing in mind these and other deficiencies of the prior art, it is therefore an object of the present invention to provide an improved patch and repair system which develops high strength quickly but still allows time for placement and working of the patch material.
It is another object of the present invention to develop a retarder for a magnesium phosphate cement system which is easily dispersed throughout the mix yet does not significantly affect early strength and other desirable characteristics of the material.
It is a further object of the present invention to provide a magnesium phosphate cement system which may be stored as a premixed, one component, dry powder until ready for use by mixture with water.
It is another object of the present invention to provide a magnesium phosphate patch and repair system with improved flow characteristics during mixing and working.
It is a further object of the present invention to provide a patch and repair system which sets in from 7 to 15 minutes at room temperature while attaining a minimum compressive strength of 3000 psi after one hour.
It is yet another object of the present invention to provide a patch and repair system which is durable, non-shrinking and develops strong bonding and adhesion with the structure or surface being repaired.
It is a further object of the present invention to provide a magnesium phosphate cement system which is useful in applications for repairing roads and other structures in which speed of repair and strength is essential.